Modeling the reactive ion dynamics in the rhizosphere of tree roots growing in acid soils. I. Rhizospheric distribution patterns and root uptake of Mb cations as affected by root-induced pH and Al dynamics

Trees growing in acid soils often suffer from nutrient imbalances and inadequate supply of base cations (Mb) but correlations between soil chemical conditions and nutritional status of forest trees are often inconclusive. Therefore, there is a need of studies that assess the Mb acquisition potential of the absorbing fine roots from the rhizosphere soil. Previous rhizosphere models, mostly implemented as a single-ion model (SIM), calculate the actual root nutrient uptake rates. But SIM often fail to reproduce measurements which is interpreted as being caused by root-induced ionic interactions. Hence, a multi-ion model (MIM) is presented which simultaneously describes the rhizospheric dynamics of H+, Al3+, Ca2+, Mg2+, K+, Na+, NH4+, NO3−, SO42−, H2PO4− and Cl− which takes into account interactions among the ions involved. In MIM the ion diffusion transport is modeled via the Nernst-Planck equation. A root-induced constant or daily-patterned water flux is assumed. The cation sorption is defined according to the cation selectivity approach. Al-solution complexes and a kinetic expression of the dissolution or precipitation of Al(OH)3(s) (gibbsite) are included in MIM. The selective nutrient root uptake is balanced by the excretion of H+ (cation uptake excess) or OH− (anion uptake excess) ions. These model features guarantee electro neutrality in the rhizosphere system but lead to ionic interactions. The objectives of this study are to calculate the rhizospheric gradients of protons, Al3+ ions and base cations (Mb), their concentration changes at the root surface (RS) and in rhizospheric sub-volumes termed as soil-root interface (SRI) and inner rhizosphere (Rh). It is hypothesized that root-induced changes of pH and the pH-dependent dissolution or precipitation of Al(OH)3(s) affect the rhizospheric concentration gradients and the actual root uptake rates (UMb) of Mb cations. In various scenarios the hypothesis is tested on the basis of different ion concentrations in the bulk soil and root uptake capacities of nitrogen and Mb ions. The simulations demonstrate that the rates of root excretions as H+ or OH− ions are determined by the preferential nitrogen root uptake as NH4+ or NO3−, respectively. A high NH4+ root uptake leads to a decrease of rhizospheric pH and a dissolution of Al(OH)3(s). An accumulation of Al3+ cations in solution and exchanger mostly on RS and in SRI is calculated due to water flux and Al(OH)3(s)-dissolution. Accumulation of exchangeable Al3+ cations cause an enhanced desorption of Mb cations in SRI if compared with SIM-results and lead to a Mb concentration increase in Rh-solution and a RS-depletion for Ca2+ and K+. MIM-calculated UMb are slightly higher compared with SIM-calculated UMb. A high NO3− root uptake leads to a rhizospheric pH increase, a depletion of Al3+ in rhizospheric solution and exchanger also at water flux caused by an Al(OH)3(s)-formation, an accumulation of exchangeable Mb cations mainly in SRI, a Mb-depletion in rhizospheric soil solution and to significantly lower UMb if compared with SIM-results. Al(OH)3(s)-induced differences in rhizospheric Mb gradients and UMb-values are determined by the magnitude of the H+/OH− root excretion rates, are highest at low Mb solution concentrations, and also occur in extremely low Al3+ bulk soil solution concentrations. An Al(OH)3(s)-formation may be inhibited at high Al3+ bulk soil solution concentrations and high H+-concentrations in solution and exchanger of the bulk soil. The range of calculated Mb depletions and accumulations in SRI and Rh correspond to the measurement results reported in the literature. It is concluded that, in contrast to SIM, MIM-simulations present asynchronous ion concentration gradients in soil solution and exchanger which include opposite concentration gradients. At NO3− surplus a high NO3− root uptake and a low availability of Mb cations may lead to wide NO3−:Mb root uptake ratios and tree nutrient imbalances.